The present invention relates to a cemented carbide body and to a method of its preparation. The invention also relates to the use of the cemented carbide body in tools.
In cemented carbides, an increase in the binder content typically leads to an increase in toughness but a decrease in hardness and wear resistance. Furthermore, the grain size of the tungsten carbide generally influences the properties in that a finer grain size gives a harder, more wear resistant, material than given by a coarser grain size, but a less impact resistant material.
In applications of cemented carbide materials in cutting and drilling tools, a combination of different properties is desired in order to maximize the efficiency, durability and tool life. There may also be different demands on the material in different parts of a product made from the material. For example, in inserts for rock drilling and mineral cutting a tough material in the interior may be desired in order to minimize the risk of fracture of the insert while a hard material in the surface zone may be desired in order to get sufficient wear resistance.
An insert of cemented carbide for mining tools is generally consumed to up to half of its height or weight during its use. The insert is subjected to impact loads which deformation hardens the binder phase gradually as the insert wears down, thereby increasing the toughness. Generally, in rock drilling and mineral cutting applications, the initial deformation hardening of the binder phase in the surface zone of a cemented carbide insert takes place during the first part, usually the first 1-5% of bit life length. This increases the toughness in the upper surface zone. Before this initial deformation hardening, during the very initial stage of the operation, there is a risk of impact damage to the insert due to a too low toughness. It would be desirable to minimize the risk of this type of early damage by providing a material which is impact resistant at the surface and the part of the material closest to the surface, during at least the initial stage of operation, without trading-off on the general requirements of sufficient internal toughness, surface zone hardness and wear resistance.
Inserts of cemented carbide for use in metal machining operations including severe discontinuous loads such as intermittent operations, or percussive operations, are subjected to high impact loads which increase the risk of damage. Also here, it would be desirable to provide a material which is impact resistant at the surface and the part of the material closest to the surface, without trading-off on said general requirements of internal toughness, hardness and wear resistance.
WO 2005/056854 A1 discloses a cemented carbide insert for drilling rock and cutting mineral. The surface portion of insert has finer grain size and lower binder phase content than the interior portion. The insert is made by placing a powder of a grain refiner containing carbon and/or nitrogen onto the compact prior to sintering.
US 2004/0009088 A1 discloses a green compact of WC and Co which is applied with a grain growth inhibitor and sintered.
EP 1739201 A1 discloses a drill bit including an insert having a binder gradient generated by diffusion of carbon, boron or nitrogen.
JP 04-128330 discloses treatment of a green body of WC and Co with chromium.